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Messages - OgreVorbis

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106
The RF Workbench / Re: Beginner class D design
« on: November 17, 2018, 1726 UTC »
1 driver per fet will be required above 3 mhz in class D.  Low RDSon is required for class D as well, I would stick to the lower R fets.  I also don't like the thin and windy traced on the drains.  This should be a large plane, not a trace.  You will have a lot of ringing problems due to the stray inductance.  Areas like this I also make sure there is no copper on the back side to create stray capacitance which will load the output unnecessarily.  Same thing with the gate traces, large and fat equals lower inductance and better waveforms.

Little things like that make a BIG difference!

+-RH

OK, that makes sense. I don't think I can fix the drain traces though. There's just not enough space on there. I think I'll attach them to a raised piece of copper clad board. I've seen this somewhere before. It shouldn't be a deal killer though, right? I mean at these frequencies, I wouldn't expect a trace like that to have any level of inductance that would cause problems. Most inductors at these frequencies use toroids, so a little loop shouldn't do much. That's just speculation though. I'm new to this and you do know more, so...

In terms of the 1 FET per driver. I did already know that, but then I saw this guy demoing his class D transmitter. It has 4 FETs and two drivers and makes 200W carrier on 40m. His 2 FET version made 100W carrier, so it doesn't seem like the drivers are limiting it. I'm also using the TC4452 and he is the lower amp TC4422.
https://www.youtube.com/watch?v=QnM2Uum-_Cs

107
The RF Workbench / Re: Beginner class D design
« on: November 17, 2018, 1259 UTC »
So I've made some progress. I decided to just make a pro PCB instead of trying to make a dead bug. The PCBs from china are cheap enough anyway.

So here is what I made. It is based on the schematic I posted earlier, but I added spaces for 4 extra FETs. Let me know what you think or if you see any errors.

I created the board in Sprint Layout which I would highly recommend. CAD software is overly complicated for these types of designs imo (and I don't know how to use it :).

I'm curious to see how well this works compared to class E. I know it won't be quite as good, but I'm more interested in bandwidth than obtaining max power from each device.

https://drive.google.com/open?id=1p_pzBloYqnlwnSdilyqJNou5V1moF8OQ

I decided to get two sets of mosfets: C2M0160120D and C2M0280120D

One has less capacitance, but lower current handling. As I understand it, the lower the capacitance, the higher the potential switching speed. I'm unsure of what is better though, more amps, or less capacitance. I find that the higher amp devices have more capacitance. So what is the higher priority? Amps could make up for the greater cap, but the less caps will make it faster. Not sure what's better.

108
The RF Workbench / Re: Beginner class D design
« on: November 14, 2018, 1737 UTC »
Thanks for your responses.

OK, so I've made some progress reading some more Class D/E schematics I could find out there. I came to find that the FAT5 from shortwaveradio.co.uk has a huge PDF with a ton of info. None of the other sites/documents I could find were nearly as thorough.

So I've compiled some info and made a schematic of my own from the bits I could find.
It is linked at the bottom. Please let me know if you see any errors. Some things I'm not clear on:

The capacitors on the drains - are they correct?
Not sure why they used 8V regulators, wouldn't 12V be better and more standard? The datasheet on these drivers also shows a slight improvement in switching speed with more volts.
I want to keep it simple to test it, so I want to try and use the old fashioned mod transformer and audio amp. I should probably know this, but: What turns ratio would I need?

https://drive.google.com/open?id=1mbt60zDB50rt65chP51dezb1KN28foh5

109
The RF Workbench / Beginner class D design
« on: November 12, 2018, 0104 UTC »
As I am building the transmitter discussed in this thread, I decided to document the progress on my blog. If you're looking to build your own class D, check it out. There is a massive archive of PDFs and schematics in there as well as an in depth explanation. I am not as experienced as some here, but I always update any inaccuracies. I also approach the topic from a novice's perspective.

http://dosaidsoft.com/wp/category/radio/class-d/

Original message:

I am trying to gain a better understanding of a class D transmitter, and while I do understand most of the concepts, I need a little clarification.

So I am probably going to start with this design: http://www.w1vd.com/40M375WclassDRev2.0.pdf
Yes, I know it's old. And I think there might be some parts left out. It looks like there is an unlabeled resistor at the gate of each FET.

First thing I am not clear on is if the exciter needs to be biphasic in order to make a class D or E amp. You can see in the schematic I linked that it has phase 1 and 2 input and it says 12V, but how many amps does it need (on the input)? Can I build half of the amp and use a normal single output DDS module? I am not sure how to make a two phase exciter. I think class D is more broadband, so I want to go with that. I'm still not even exactly sure what the difference is between D and E despite them being described so often. I never find a clear difference.

So when I have the amp and exciter built, I think I'll start trying to drive it with a mod transformer instead of PWM just to start because it's easier. So how many ohms do I need to match right at the drain of the FETs? Is it 50? No, probably not because that's after the matching right? So if I use a generic audio amp, what transformer ratio do I need? I think I'll use a microwave oven transformer. I have one that has all coils removed, so it's just a bare core I can wrap around.

If I need a two phase exciter. Where can I find one? I do know about the classeradio.com guy, but he only sells the PWM and not the exciter. He didn't respond anyway. Probably because I'm not ham.

And, yes, Stretchyman, I know you're here too. Your transmitter is tempting, but I don't want to just buy it without understanding how it works because that gets annoying in the long run.

Would you mind explaining a little to me? Mostly on how your DDS module works (I assume you use DDS). Is it a two phase thing or is it essentially just a 9850 and a PIC on a board - like what you can get on eBay? I would really like to see the schematics and also if you can clarify how the exciter module works. If you'd like a small payment for your schematics, I'd be willing. I'd just buy the whole thing if you can tell me how to change it for more bandwidth.

If I was building it myself I'd have used Class D for more bandwidth, so it's not perfect from my standpoint. What about your amp makes it Class E and could it be changed to D? I've seen a class D 100W amp that was using two SiC devices and it works 3-8 MHz. What makes yours only 200 Khz range?

I know that's a lot of questions and I am being very critical of the design. I know it is better than most others out there given that it's using the newest GaN and drivers. I hope you understand I'm am trying to learn w/o just buying stuff and wiring it up. That's too easy for this hobby and it makes it boring.

110
The RF Workbench / Re: PWM a BLF188XR?
« on: November 10, 2018, 1138 UTC »
BTW: I have a 20W PEP amp with 2x RD15HVF1 and the BLF188 (which I haven't finished constructing yet). I plan to drive the BLF with the former at reduced power.

I am using the 9850 DDS board I got from Germany on eBay (forgot the name). It has AM modulation built in. I think it is rf2017.

I will test it out soon, but I need a water cooling block and I'm trying to decide if I should buy or build one. I think I'll build it because I spent a lot of time looking for a good one. Not just size wise, but also with known thickness and tube pattern, so I know I am not going to puncture it when I drill the holes to mount the BLF. I'd rather not used forced air. It will just be bulky and loud. I already have a good radiator which should be more than capable.

111
The RF Workbench / Re: PWM a BLF188XR?
« on: November 10, 2018, 1130 UTC »
No it won't work as its a LINEAR AMP.

As soon as you reduce the voltage it will cease to function!

You need Class E which is way more efficient, simpler, cheaper etc etc.

They use those devices you quoted as they are modern, i.e. available, are cheap and are very easy to drive and have a low on resistance. There's no need to use GaN as they're for much higher (RF) frequencies. No real need to use SiC either unless the voltages are very high. Any crappy ol' FET will work as a PWM output as the frequency is only 200KHz or so....you just DONT want Ohms of on resistance as with highish current flow you'll drop volts and generate a lot of heat. SiC FETs can be use with high voltages keeping the current low, I2R is what generates loss.

With a linear amp you're best directly modulating the DDS and amplifying that to 3W to drive the linear. If you check my old posts I mention directly modding a AD9850 and there's a good article on the web about such..

Bought the same amp as you to do said build! You may beat me to it! Was also going to make a 3W SSB generator to drive it too!

Keep us posted!

Str.

Thank you for the response, and if you don't mind, see my other post in your thread about your 400W PWM.

What I meant to say about the BLF188 is: Why don't they use it in Class E configuration? Is it not possible? Is it the on resistance you where talking about?

112
The RF Workbench / PWM a BLF188XR?
« on: November 10, 2018, 0744 UTC »
This may sound like a stupid question to those who know more about RF engineering, but...
Why have I never seen an RF MOSFET PWM modulated or other high level modulated?
They are always used as linear amplifiers.

I have a BLF188XR amplifier board and I was wondering if it is possible to modulate it this way? Is low level modulation the only option with this?
Why does every PWM use GaN or SiC or those IRFP (whatever they're called)?

113
For Sale / Wanted / Barter / Re: 400W RF GENERATOR
« on: November 10, 2018, 0531 UTC »
I'm am interested in this transmitter, but I'm more interested to see the schematics than to buy it. I've learned I will always want more power and upgrades, and for that, it's better for me to understand the design first. Would you be willing to offer the schematics?

I'm also curious about the bandwidth of this design. Could a DDS module from ebay with 9850 and free tuning 2-30 MHz be used instead of channels? Of course, it probably can't go all the way to 30 MHz. You say 200 KHz segment, so it's narrowband (ie not balun match)?

114
MW Free Radio / UNID 1720 AM ~0445 UTC 11 Oct 2018
« on: October 11, 2018, 0532 UTC »
Did anyone hear it?
Begins with a carrier on and off and then music.

115
General Radio Discussion / Re: expanded FM band in Americas
« on: August 30, 2018, 0442 UTC »
I remember reading somewhere that the FCC thought about extending the Fm band after analog tv was shut off but there wasn’t enough public interest.

I've heard people say multiple times that since analog TV is gone, then those frequencies should be free. Maybe I'm misunderstanding, but I always thought that the digital stations used the same frequencies for broadcasting that the old analog stations used. Am I correct? If not, then what frequencies are they using?

116
Yeah, no kidding! Just how hard would it be to write up software for SDR's to be able to have C-QUAM capabilities? I am the last person to know this since I almost failed out of computer electronics in college, since I hated machine language.

For near the same price as this you could get a HackRF. With that, you can pretty much modulate any type of signal you want. Sure you could do c-quam with it, but I don't know how. It doesn't require assembly language programming. It uses the gnuradio software which is like a flow chart. Not sure how well it works on AM band though. Some say it works down to 10 MHz and others say 1 MHz.

117
MW Free Radio / Re: UNID 1700 AM 0135 UTC 23 June 2018
« on: July 20, 2018, 1933 UTC »
Not sure what this is booming into a DC-based SDR right now....coming in clear, a bit of periodic static.  Big band/jazz music, on 1700 khz.  I can't find any station on that frequency....perhaps a pirate? Here's a recording of what I heard. 

https://drive.google.com/open?id=1bSk42mNsCK5CYvLXDtgvcIxu9LDs8LK5

Little help, anybody?

There's only two DC based SDRs that I'm aware of and one of them always has something playing on 1700. Did you see the tag under it? It says the guys callsign and that it's a part 15 station.

118
FM Free Radio / Re: Antenna Height vs Gain
« on: July 20, 2018, 1908 UTC »
Can you guys give me your general thoughts on this?  I know there is no right answer.

I've finally built a circular polarized antenna to try to get a more consistent signal no matter what type of receiver.  My original idea was to build a second CP antenna and feed it in phase a wavelength below the first.  Now I'm having second thoughts on the two-bay idea.

I'm in a rural area and my existing ground plane antenna is nearly 100 feet above the terrain.  It doesn't quite clear the treeline, but it's as high as I'm likely to get without a real tower, which isn't going to happen.  It's a good transmitter site, but despite some pretty cavalier power levels, I just can't get the signal to my house.  I think I'm terrain-limited rather than power limited.

Here's my thought.  I'm not really comfortable using high power long-term.  It seems like asking for trouble.  I'd like to get the most out of a lower output power, but if adding another CP antenna will just lower the center of radiation by one half wavelength, would you expect to get any additional range out of the higher gain?

All things considered, I can just run more power to a single antenna at the same height.

I have some decent experience with this although I've never tried a CP antenna. I've tried ground plane antennas, dipoles, and the "dominator" antenna (which is a 5/8 wave with interesting matching). Anyway, in a hilly location, I've actually found the ground plane type antennas work better at a lower height than anything with higher gain. The lack of gain almost compensates for low height (I'm considering 100ft - low). It shoots the signal up more. Yes, some of it will be lost in space, but on the low end of the FM band, there's some bending that can occur and the GP will cause the signal to go over hills a bit better.

With the amount of power you're using, you should be getting farther than that. My guess is the transmitter site is just in a bad location. Something I've thought about doing, was putting a solar powered TX on a hill nearby with a directional wifi dish for streaming audio and controlling the transmitter. You could get away with only 15W on a hill and go much farther than 150W on the ground. Height is pretty much everything, not power. I did a test with a 30W transmitter on a mountain that I lugged up with a car battery (wasn't easy). The signal went about 30 miles.

Another thing to consider if you just want to make it from the site into town is a yagi.

One more thing - Don't use stereo. Give mono a try. When you're really close to the TX, it won't sound as good, but I came to find that with my setup, using mono made the signal much clearer longer distances away. And at those distances, nobody is going to be hearing the advantage of stereo because receivers default to mono with low signal level anyway.

Are you getting multipath distortion? When you drive listening to the signal, does it pop in and out. Sounds  pretty clear, but with bursts of static. If yes, use mono or SSB stereo mode instead of normal stereo.

119
Equipment / Re: Starting a pirate radio in Spain
« on: July 20, 2018, 0257 UTC »
I just discovered two things about that transmitter today.

You will need a -10db attenuator on the input to the amp because it drives it to almost full power even when you adjust the power pot. You won't be able to modulate it without (only CW).
You should put a simple RC audio filter on the input because it might create spurs from random high frequencies.

120
Equipment / Re: Starting a pirate radio in Spain
« on: July 19, 2018, 1023 UTC »

Dear OgreVorbis,
Thanks for answering all of my questions and helping me shape much better this project. I've definitely learned a lot. I'll report back when I have everything up and running!

Thank you very much and 73!
Best regards,
Diego

You're welcome. And if you didn't see it, the REL Map button in VOACAP seems to be the most straightforward. The rest of it is a little confusing.

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